This invention is related generally to holographic recording and playback systems and is more particularly concerned with a holographic system for recording on and retrieving video information from a rotatable disc of material.
Since the earliest days of television, a means has been sought by which television programming may be recorded in a convenient and easily mass reproducible format. Throughout the period of time in which a solution to this problem has been sought, the phonographic recording technique for audio information has been present and developing. Therefore, it is not surprising that a very large number of the proposed solutions have utilized a disc format for the recording medium.
Although the disc format has been widely considered, the manner in which the video information is impressed onto and retrieved from the disc has varied very widely. In the earliest attempts, a standard phonographic technique was utilized. This was possible because at the time a 30 line, 30 frames per second, non-interlaced system requiring a bandwidth on the order of 10 to 15 kHz was utilized. Present day television in the United States requires a bandwidth of about up to 5 MHz in a 525 line, 60 fields (30 frames) per second, interlaced system. The information capacity of the system is thus required to be at least 300 times greater than is capable of being recorded in a standard phonograph format.
Among the modern video disc proposals, one technique is closely analogous to that of conventional audio recording. A very thin plastic disc has a single spiral groove therein much like a standard phonograph record; however, the grooves are much finer and are much more closely spaced, about 140 grooves per millimeter. The disc is revolved at a rate of 1800 revolutions per minute, i.e., once for every television frame. The modulation in the groove is up and down instead of lateral as in standard phonographic recordings. The sound is carried on the same groove by pulse position modulation. The information is retrieved by a pressure sensitive piezoceramic pickup unit.
Another similar technique has been announced in which a metallically coated sapphire stylus is caused to ride in a modulated groove in a video disc and senses variations in the electrical capacitance between its coating and a metallic covering on the video disc.
Some other techniques utilize an optical data pattern in a single spiral track on a video disc. In such systems, it is again conventional to utilize a rotational velocity of 1800 revolutions per minute. The information for the reproduction of the television image is recorded as a succession of short grooves or pits of variable length and repetition frequency. If a spot of light falls on the surface of the disc between two of the pits, most of the light will be reflected. If, on the other hand, the spot falls on one of the pits, the light will be deflected by diffraction at the pit in such a way that most of it is not returned to an objective lens. In this manner, the intensity of the light reflected through the aperture of the lens is modulated by the pattern of pits and grooves. The grooves are necessarily very closely spaced, and extremely precise optical tracking is a requirement of the system.
This system does not utilize a stylus for tracking the groove as in conventional audio recording. The tracking is accomplished by ultra-precise positioning of the pickup head above the groove in the surface of the disc. This tracking technique requires feedback-guided servomechanism pickup arm drives together with delicate electronics and critically adjustable mechanical mechanisms. Such an electromechanical system is not generally believed to be compatible with low cost, consumer oriented video playback systems.
Holographic systems have also been proposed for use in video disc technology. One such system is shown in British patent specification No. 1,139,955. In this holographic system, a series of holograms is recorded on a disc of material as surface relief holograms. The series of holograms are arranged in a spiral on the surface of the disc. Each successive individual hologram constitutes all of the necessary information for reconstructing one complete television frame. In other words, the holographically recorded video disc is more analogous to a strip of motion picture film than to a phonographic audio recording.
In principle, this technique appears fairly suitable for use in the retrieval of video programming. However, several serious problems arise with this system which preclude any possibility of it becoming a commercial reality. Primary among these problems is a need for a high resolution, sensitive vidicon or other two-dimensional image sensor in the playback apparatus to convert the complete frame of information to serial information accepted by a conventional television receiver. In some later versions of the system, two or more registered vidicons are required in order to achieve playback. Obviously, when the electronics required for such a system are added to the normal complexities of mechanical synchronization and optical readout, a system is presented which cannot conveniently be reduced in cost to a point where commercial feasibility is realizable. Furthermore, this video disc cannot be recorded in real-time, but must first be electrically encoded and recorded on film before transferring the information to the video disc.
Other nonvideo disc systems have also utilized holograms for recording and reproducing television signals. For example, U.S. Pat. No. 3,657,473 teaches a system in which a modulated data beam is scanned through a range of intersection angles with a strip of film at the horizontal sweep rate. The intersection angles are in a plane which is generally normal to the film and which contains the long dimension of the film strip. At the same time, both the data beam and reference beam are swept across the surface of the film strip at the vertical sweep rate in a direction which is orthogonal to the plane of the horizontal sweep. Simultaneously, the film strip is moved slowly in the direction of sweep for the horizontal. During playback, the reference beam is occluded and the data beam is swept across the film strip in the original manner. While this system, with the film packaged in cassettes, is perhaps an alternative to the video disc type of system, it suffers greatly in that the cassettes of holographically recorded material are not as susceptible as embossed one-piece disc to mass production at prices within a range which might be considered suitable for a home entertainment market. Furthermore, registration between the film strip and the complex optical scanning apparatus is difficult to achieve and may preclude such a system from achieving commercial acceptance.